A hybrid four-wheel-drive vehicle may have either the front or rear wheels driven by an engine, and the other wheels driven by a motor. Some hybrid vehicles provide a motor/generator connected to the front or rear wheels to drive these wheels by turning on the motor when necessary, or conversely, capturing rotational energy of these wheels by converting the rotational energy to electric energy to be stored, e.g., during braking. However, general practice requires that the power distribution to the wheels from the motor/generator is performed after reducing the rotational velocity of the motor/generator using a two-step parallel gear reducer. However, a transmission with a two-step parallel gear reducer causes the motor to be offset radially from an axis of a driveshaft. This results in a radially large drive system including the motor, which limits the practical application of such a design.
It has been proposed to provide an electrical motor in a coaxial arrangement with a rotational axis of wheels in order to conserve space by creating a hollow space at the center of the electrical motor through which a driveshaft can be located.